Device for determining the composition of sodium-lead alloy



Nov. 8, 1955 'r. R. VICK ROY 2,722,333

DEVICE FOR DETERMINING THE COMPOSITION OF SODIUM-LEAD ALLOY Filed Jan. 21, 1952 IO-OO To Plummet INVENTOR.

THOMAS R. VIGK ROY 3.4 mud A TTORNE Y.

United States Patent DEVICE FOR DETERMINING THE COMPOSITION OF SODIUM-LEAD ALLOY Thomas R. Vick Roy, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Application January 21, 1952, Serial No. 267,483

2 Claims. (Cl. 73451) This invention relates to a device for recording the composition of a molten sodium-lead alloy, and more specifically to a device responsive to minute changes in the specific gravity of such compositions.

An alloy of lead and sodium in substantially equal molecular proportions corresponding to the formula NaPb is used in large quantities in the manufacture of tetraethyl lead. The sodium content of the alloy must not deviate by more than plus or minus 0.015% from an empirical standard, determined chemically usually about 10.015% sodium, or the yield becomes unsatisfactory. In practice, molten lead and molten sodium are weighed and mixed together in large manufacturing pots. Control samples are taken periodically to make sure the alloy does not vary from 10.015 sodium more than that indicated above. If the analysis shows such a deviation, more of the deficient metal is added to bring the alloy to the correct composition, and a sample is again taken and analyzed. When the alloy has been adjusted to the exact required composition, the charge is then released for the manufacture of tetraethyl lead.

In sampling the alloy for analysis, a small amount is Withdrawn from the pot, cooled and pulverized. A suitable amount is weighed out and treated with water to obtain the sodium in the form of the hydroxide, which is then titrated with standard acid. Because of the hazards involved in handling such corrosive material at high temperatures, and because of the precautions necessary to avoid exposure of the samples to air, the sampling and analysis requires expert technique and at least an hour and a quarter, even when such analysis has become a routine operation. Since the alloy must remain in the manufacturing pot until its composition has been adjusted within the very narrow permissible range, several additions of sodium or lead may be necessary, each of which must be followed by an analysis. It will be apparent, therefore, that several hours may normally be required for arriving at the correct composition of the alloy. Obviously, if a method could be devised which would reduce the time for carrying out these operations, the amount of alloy-manufacturing equipment required for tetraethyl lead installations could be considerably reduced.

Many suggestions for overcoming the difficulty have been proposed, such as those based on electrical resistivity, X-ray difiraction, melting point and specific gravity. These difficulties have been rendered more troublesome because of the high temperatures involved (above 400 C.), in handling the sodium-lead alloy, and because of the reactive nature of the alloy, particularly its extreme sensitivity to water and oxygen. The problem is even more difficult because of the very unusual degree of accuracy and reliability required in determining the true composition of the alloy. In view of these facts, it appeared highly improbable that a recorder based on the abovementioned properties offered a satisfactory basis even for preliminary study. After considerable research on the problem, it was found that a small variation in the percentage composition near the point reflected a rela- Patented Nov. 8, 1955 tively large variation in the specific gravity of the alloy above its melting point.

It is, therefore, an object of the present invention to provide a device for indicating or recording the composition of a sodium-lead alloy with great accuracy based on the variation in specific gravity measurements at about 418 C. Another object is the provision of a device which records continually the percentage composition of such alloy with accuracy and reliability. A still further object is the provision of such a device which requires a minimum of attention and eliminates frequent routine chemical analysis of the alloy. Other objects will appear as the description of the invention proceeds.

These objects are accomplished by a device for determining the specific gravity of the sodium-lead alloy under certain specific conditions. These involve the use of a specially designed plummet having a density greater than 6, preferably between 6 and 10 g./cc., but preferably only slightly above that of the molten alloy. This plummet is suspended entirely Within the molten alloy by means of a fine wire attached to a beam of a counterweighted balance which responds to changes in the apparent weight of the plummet by reason of changes in the density of the alloy.

In the drawing, Figure 1 represents a diagrammatic elevation view of the apparatus in which the balance is shown as a unit without detail. Figure 2 is a detailed diagrammatic view of the balance mechanism. The numerals refer to the same parts in both figures.

In Figure 1, the manufacturing pot 1 contains the sodium-lead alloy 2, the surface of which is indicated as 3. A housing 4 is providedwith perforations 5 to allow the alloy to permeate throughout the housing without appreciable upward or downward currents.- This housing is also provided with a cover 10 and an inlet 8 for introducing nitrogen or similar inert gas to prevent reaction between the alloy and the oxygen or moisture in the atmosphere. Centrally located in the cover of the housing is a fine hole 9 through which a Wire 7 passes. The plummet 6 is suspended by the wire 7. The other end of the wire leads directly upwardly to a balance mechanism shown diagrammatically as 11.

Figure 2 is a more detailed diagrammatic view of the balance mechanism, which consists essentially of a beam 12 balanced centrally on knife edge 13. The wire 7 is attached to one end of the balanced beam and on the other end a counter-weight 14 is attached. The beam is also provided with a lower span-adjusting Weight 17 which may be moved up or down on screw 18. Stops 15 and 16 are provided to regulate the amount of the beams swing. The beam is also provided with static balance adjustments shown as 22, and a pointer fixed to or otherwise actuated by the beam designed to swing across the scale or chart 20 calibrated in percent sodium ranging from 9.75% to 10.25%. If a continuous record of the sodium content is desired, any standard recording device may be used, such as a rotating cylindrical chart shown diagrammatically, which may rotate about the axis 21. The pointer 19 in such case is provided with a pen (not shown} to record the values on the scale 20. Other recording charts, such as rotating discs or moving scales of various shapes may be used. For simplicity the pointer is shown fixed to the beam, but in practice it is actuated through suitable levers or linkages which amplify the swing to make legible record on the chart.

The plummet 6 is a very important part of the invention. Preferably it should have a density and coefiicient of expansion not very different from the NaPb alloy to reduce errors due to changes in temperatures. It has been found that steel may be used satisfactorily for the thick parts because of its ease of fabrication, strength, moderate cost and availability. Molybdenum is particularly useful where extreme resistance to corrosion is necessary, such as in the fine suspension wire. The plummet, as well as the wire, could be made of molybdenum or tantalum, either entirely or only its outer layer if cost, fabrication and machinability were not of particular consequence. The sensitivity of the measurement of density, other things being equal, depends inversely upon the difierence in density between the plummet and liquid, hence the sensitivity may be increased by decreasing the average density of the plummet, such as by manufacturing it with a hollow center, but it must have a density sufiiciently high to sink in the molten alloy when unsupported. The density of the molten alloy at 4l8.3 C. is 5.958 g./cc. at 9.75% Na in the alloy and 5.811 g./cc. at 10.25% Na.

In the operation of the invention, the manufacturing pot provided with the device described above is filled with the alloy of sodium and lead in equi-molecular proportions. The temperature is raised or lowered by any known means and the charge is stirred vigorously to provide homogeneity. The balance is adjusted to indicate the correct percentage composition of sodium at the pointer or on the recording drum. The calibration of the recording device or scale is first accomplished by analyzing the alloy chemically at various points between 9.75 and 10.25% sodium in the alloy. Thereafter, variations in the percentage of sodium in the alloy are recorded with a high degree of accuracy and reliability on the recording scale. It has been found that once the scale is calibrated, the chemical analysis agrees with the recorded value within about i0.015%. The accuracy of the device is checked occasionally by a chemical analysis of the alloy to detect any deviation due to corrosion of the plummet and suspending wire.

The main advantage of the present invention is that it solves a problem of eliminating time-consuming chemical analyses which heretofore was considered incapable of practical solution by the commonly available devices for recording percentage composition based on electrical resistancy, melting point, X-ray diffraction or density. It is surprising that slight variations in density can be detected with a plummet so reliably in such a heavy alloy. Another advantage of the invention in addition to the elimination of chemical analyses is the relative simplicity of the device as compared with complicated recorders sometimes used.

It is apparent that many widely difierent embodiments of this invention may be made without departing from the spirit and scope thereof, and therefore it is not intended to be limited except as indicated in the appended claims.

I claim:

1. The combination of a melting pot for containing a molten alloy of about equi-molecular proportions of sodium and lead, there being a closure for the top of said pot having an aperture therein a plummet having a density between 6 and 10 g./ cc. for immersion in the said molten alloy, a balance provided with a centrally pivoted beam, a pointer actuated by the said beam and a scale indicating the composition of the said alloy, a fine wire for suspending the immersed plummet from one end of the beam, a counterweight suspended from the opposite end of the beam, a foraminous housing extending into said pot from said aperture and around the said plummet and means for introducing an inert gas into said housing above the surface of the alloy in the melting pot.

2. The apparatus of claim 1 in which the density of the plummet is slightly greater than that of the alloy.

References Cited in the file of this patent UNITED STATES PATENTS 1,272,605 Baker July 16, 1918 1,628,050 Kraus et al May 10, 1927 1,787,132 Van Orsdale Dec. 30, 1930 FOREIGN PATENTS 20,672 Germany Jan. 17, 1883 fi -w 

1. THE COMBINATION OF A MELTING POT FOR CONTAINING A MOLTEN ALLOY OF ABOUT EQUI-MOLECULAR PROPORTIONS OF SODIUM AND LEAD, THERE BEING A CLOSURE FOR THE TOP OF SAID POT HAVING AN APERTURE THEREIN A PLUMMENT HAVING A DENSITY BETWEEN 6 AND 10 G./CC. FOR IMMERSION IN THE SAID MOLTEN ALLOY, A BALANCE PROVIDED WITH A CENTRALLY PIVOTED BEAM, A POINTER ACTUATED BY THE SAID BEAM AND A SCALE INDICATING THE COMPOSITION OF THE SAID ALLOY, A FINE WIRE FOR SUSPENDING THE IMMERSED PLUMMET FROM ONE END OF THE BEAM, A COUNTERWEIGHT SUSPENDED FORM THE OPPOSITE END OF THE BEAM, A FORAMINOUS HOUSING EXTENDING INTO SAID POT FROM SAID APERTURE AND AROUND THE SAID PLUMMET AND MEANS FOR INTRODUCING AN INERT GAS INTO SAID HOUSING ABOVE THE SURFACE OF THE ALLOY IN THE MELTING POT. 